In Ms. Marshall's class, individual students can earn tickets for engaging in academic work and for demonstrating positive classroom behavior. At the end of the week, each student can exchange their tickets for a reward (e.g., 10 minutes of computer time, a pencil, etc.). This is an example of:
 A. 
 observational learning
 B. 
 classical conditioning
 C. 
 a token reinforcement system
 D. 
 group consequences